add_filter( 'dynamic_sidebar_params', 'change_widget_titles', 20 );
function change_widget_titles( array $params ) {
// $params will ordinarily be an array of 2 elements, we're only interested in the first element
$widget =& $params[0];
$widget['before_title'] = '<div class="widget-title">';
$widget['after_title'] = '</div>';
return $params;
}